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if i'm not wrong, R3 is part of a high current protection. The idea is to find the resistance value necesary to turn on Q5 when your output current is higher than what you want so that when Q5 passes to active region, Q1 also get's on and the excess of current flows through Q1's collector. In this case your output current is 200 mA, so the current through R3 would be 200 mA plus the small amount of current that flow's through R2 to keep the Zener ON. Your protection should start working when that current is a little bit higher than this value (200 mA + Izener). In this case R3 would be VbeonQ1/(200mA + Izener). (assuming Vbeon=0.7, R3=0.35 ohms APROX).
Your input voltage should be as low as it can be, so the consumed power on your circuit is as low as possible. The limit of this value is given by the dropout voltage which would be Vd2 + VcesatQ3 + VR3max (the minimum voltage needed to have all your circuit's components operating as you want them to work), Knowing that Vdcin ≥ Vout + Vdropout (assuming Vd1=0.7, Vcesat=0.2, Vdropout = 0.7 V + 0.2 V + 0.7 V = 1.6 V making Vindc ≥ 9.6 V) Remember no to use values to close to the limit cause your input voltage usually has ripple.
Finally to know the voltage value of the Zener all you need to know is the voltage output value. In this case Vout = 8 V and also Vout = Vzener + VbeonQ3 + Vd1. (with the previous assumptions Vzener would be 9.4 V). Remember to adjust the value of R2 so that it's current (which is the same as the one flowing through the Zener) is enough (but not to big) to keep the zener on.
Two basic series linear regulators are in the attachment below. One of them has fold back current limiting, the other is even simpler the diode is for temp comp. These aren’t exactly high precision but they should give you a starting point.
Two basic series linear regulators are in the attachment below. One of them has fold back current limiting, the other is even simpler the diode is for temp comp. These aren’t exactly high precision but they should give you a starting point.
Thanks max0412,
Plese tell me the process of doing paramertic sweep in pspice.ie how did you bring that r7{VAR} in curly bracket.Can we get Vin Vs Vout or R7 Vs Vout grahph in pspice simulation?
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